1. Field of the Invention
This invention pertains to mechanisms that transport and move semiconductor wafers during their fabrication process. More particularly, this invention relates to mechanisms that remove wafers from a wafer holder such as a wafer cassette and transfer the wafers to a destination location for subsequent operations.
2. Background
During semiconductor fabrication, ingots are formed and sliced into wafers. The wafers proceed through many steps that require the wafers to be temporarily stored, transported and positioned. Wafer cassettes are often used for some of the temporary storage and transportation of wafers. Cassettes are typically slotted containers which are open at the top and which have four side walls and a bottom. Inner surfaces of two opposing side walls are fluted to create slots. Individual wafers are each positioned within a respective slot and are held therein, separated from adjacent wafers. When a cassette is fully loaded, the wafers are located in respective slots and spaced apart from one another so that wafers do not abut one another. Cassettes are therefore useful for storage and transportation of a group or stack of wafers.
Efficient fabrication requires that wafers be quickly and securely transported from one station to another. And, because wafers are expensive and fragile, the wafers must be handled in a manner to prevent damage or breakage.
U.S. Pat. No. 5,299,901 describes prior art mechanisms that remove wafers from a cassette and move them to a wafer "boat." Early mechanisms used mechanical tweezers or vacuum tweezers to grip individual wafers and remove them from a cassette and move them. However, these gripper mechanisms could damage some of the wafers. A later transfer mechanism used a "pusher" to push individual wafers up and out of a cassette. A pair of clamps would then grip the wafer and move it.
In the wafer transfer mechanism described in the '901 patent, a cassette is attached to a platform and inverted. A wafer boat is located below the inverted cassette and a "lifter" is raised into position below a wafer so that the wafer comes to rest on the lifter. The lifter is then lowered and the wafer, resting on the lifter, descends into the boat waiting below. The wafer is thus transferred from the cassette to the boat.
The wafer transfer mechanism described in U.S. Pat. No. 5,299,901 transfers wafers between devices that can be moved and located vertically atop one another. It does not appear capable of transferring a wafer to another device that is located in a horizontal plane; nor does the transfer mechanism appear capable of placing a wafer on horizontal surface. These are some of the limitations that prevent this prior art transfer mechanism from efficiently transferring wafers to some types of work stations and horizontal conveyors.
Another known wafer transfer mechanism has an elongated arm which is inserted beneath a wafer in a cassette. The arm is removed from the cassette with the wafer coupled to the arm using a vacuum. The arm is then rotated about its longitudinal axis to turn the wafer for delivery to a second location.
Although these prior art wafer transfer mechanisms exist, a need remains for an improved wafer transfer apparatus.